The invention disclosed and claimed herein pertains generally to the field of acoustic signal source detection, by means of a beam forming array comprising an interconnection of discrete acoustic sensor elements. More particularly, the invention pertains to the above field wherein it is essential to know the positions of respective sensor elements in order to accurately interpret data provided by the element array. Even more particularly, the invention pertains to a system in the above field which periodically samples the velocity of the leading element of a beam forming array as the array moves through an acoustic environment, such velocity samples being employed by a processor, along with array drag forces, to determine array element positions.
Flexible linear arrays of acoustic sensor elements, such as hydrophones or the like, are extensively used in an acoustic environment such as an ocean body to locate the position or bearing of an acoustic signal source contained therein, and also to provide signal-noise gain in an acoustic detection system. Such arrays generally comprise a number of sensor elements which are joined together by flexible cable or the like at regular intervals, and which are selectively moved through the environment, reception capabilities of individual sensor elements together forming a beam or search pattern. In order to beam form, i.e., to process signals to determine the bearing of a signal source and to provide signal-to-noise gain, it is essential to know the positions of array elements in relation to one another. In the past, such information has been provided by making the assumption that the array is in a linear configuration, whereupon the positions of the elements in relation to one another is easy to determine.
As is well known in the art, the above assumption is often invalid, such as when the array or vessel is exposed to strong lateral currents or winds, or when a portion of the array sinks to a lower depth than another portion thereof, or when the vessel is not on a straight path. If the actual positions of the sensor elements are sufficiently different from the positions which are respectively assumed therefor during beam forming, the signal-to-noise gain in the beam forming process may not be adequate for target source detection or for calculation of target source bearing in an environment which contains a great deal of noise.
In order to determine the actual positions of the elements of a linear array at present, a high frequency pinger or the like may be located in proximity to the array, and the time delays in pinger signal detection may be compared for each of the elements. However, the use of such active signal devices may be undesirable where there is a possibility that the pinger signals will be detected by one for whom they are not intended. Also, it is necessary to know the location of the pinger. Alternatively, speed and heading sensors may be placed at each of the elements along an array, and information provided thereby used to estimate respective element positions. However, in an array having a length which is on the order of a mile, a large number of such sensors must be distributed along the array, creating substantial maintenance and calibration problems.
In the present invention, a single speed sensor device and a single heading sensor are provided to periodically sample the speed and heading of the leading element of a line array of acoustic sensor elements, as the array is drawn through an acoustic environment. Such information is coupled to a processor and employed thereby, together with various parameters prestructured into the processor, to closely approximate respective sensor element positions at the end of each sampling interval. It is anticipated that the differences between actual and estimated positions of respective elements will decrease as the number of sampling intervals increases. It is further anticipated that such differences can be reduced to within a limit which enables the array to be used to monitor acoustic signals of selected frequencies, a particular limit being required for a particular frequency.